1. Field of the Invention
The present invention relates to a laminate of electret films which is composed such that the films are in intimate contact with each other, with the surfaces of the same polarity facing each other.
2. Description of the Prior Art
Electrets, which are charged or polarized for a long period of time without being supplied with any external energy, are being used or going to find use in the areas of acoustic parts, filters, and medical supplies. They are required to have a high surface charge density and keep the charge for a long period of time. However, electrets tend to lose the charge when they are left in the air. Moreover, the surface charge density decreases still more rapidly when there exist moisture and dust in the air. Thus, it is very difficult to keep electrets stably charged for a long period of time. For this reason, it has been a common practice to fabricate and store electret articles in a place where there is a minimum of moisture and dust.
According to Japanese Laid-open Patent No. 28698/1977, an electret of multilayered laminate structure keeps the charge for a long period of time. Even such an electret does not keep, without difficulty, the initial high charge density at the time when it is formed.
There is known a laminated article composed of an electret and a non-electret material. In such an article, the non-electret material acts merely as a reinforcement and does not effectively prevent the charge density of electret from decreasing.
In the process of forming an electret, one side of the material usually becomes positively charged and the other side, negatively charged. However, in the case of electrostatic acoustic transformers such as microphone, it is required that both sides be charged with the same polarity. According to U.S. Pat. No. 3,924,324, it is possible to obtain a film having the surfaces charged with the same polarity by placing an electret film on a metal plate and heating the metal plate. This method, however, is not preferred to obtain an electret of single polarity having a high charge density, because heating an electret leads to the decay of charge.
Accordingly, it is an object of the present invention to provide an electret article which stably keeps the charge density over a long period of time.
It is another object of this invention to provide an electret article which loses a minimum of its charge density even after it is placed in a humid or dusty environment for fabrication or storage.
It is a further object of this invention to provide an electret article having a high charge density.
It is an even further object of this invention to provide an electret article having charge of the same polarity on both surfaces.
Other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings.